As described by F. Capasso et al. in Solid State Communications, Vol. 102, No. 2-3, pp. 231-236 (1997) and by J. Faist et al. in Science, Vol. 264, pp. 553-556 (1994), which are incorporated herein by reference, a quantum cascade (QC) laser is based on intersubband transitions between excited states of coupled quantum wells and on electron transport as the pumping mechanism. Unlike all other semiconductor lasers (e.g., diode lasers), the wavelength of the lasing emission of a QC laser is essentially determined by quantum confinement; i.e., by the thickness of the layers of the active region rather than by the bandgap of the active region material. As such it can be tailored over a very wide range using the same semiconductor material. For example, QC lasers with InAlAs/InGaAs active regions have operated at mid-infrared wavelengths in the 3.5 to 17 .mu.m range. In diode lasers, on the contrary, the bandgap of the active region is the main factor in determining the lasing wavelength. Thus, to obtain lasing operation at comparable infrared wavelengths the prior art has largely resorted to the more temperature sensitive and more difficult-to-process lead salt materials system.
More specifically, diode lasers, including quantum well lasers, rely on transitions between energy bands in which conduction band electrons and valence band holes, injected into the active region through a forward-biased p-n junction, radiatively recombine across the bandgap. Thus, as noted above, the bandgap essentially determines the lasing wavelength. In contrast, the QC laser relies on only one type of carrier; i.e., it is a unipolar semiconductor laser in which electronic transitions between conduction band states arise from size quantization in the active region heterostructure.
Although relatively short wavelength (e.g., .ltoreq.3 .mu.m) diode lasers routinely operate continuous wave (cw) at room temperature and above, QC lasers at present do not. Rather, at these relatively high temperatures they are operated only in a pulsed mode due to their relatively low wall-plug efficiencies (.ltoreq.1%) and their relatively large electrical power dissipation (.ltoreq.10 W). The latter originates from the high operating voltage and/or current (e.g., 5-10 V at 1 A). The highest reported cw operating temperature for QC lasers emitting at wavelengths of 5-8 .mu.m is .about.150-160 K. However, for many applications, e.g., high resolution gas sensing, it would be highly desirable to have a QC laser capable of operating cw at higher temperatures so as to reduce the electrical power requirements on the cooling system used and to enhance the laser performance.
One of the factors that significantly affects the highest temperature at which a semiconductor laser can operate cw is the heat sinking technology used to extract heat from the active region of the device. The laser may be mounted on a heat sink either with its substrate side down (i.e., with the relatively thick substrate mounted on the heat sink) or with its epitaxial side down (i.e., with the relatively thinner epitaxial region, grown on the substrate, mounted on the heat sink). The latter approach has the advantage that the active region, which is formed in the epitaxial region, is placed in closer proximity to the heat sink than it would be in the case of substrate-side mounting. Consequently, heat extraction from the active region is enhanced, and higher temperature and higher power operation is possible. Illustrative of fairly current investigations into epitaxial-side mounting of semiconductor lasers are papers by Bewley et al., Appl. Phys. Lett., Vol. 74, No 8, pp. 1075-1077 (Feb. 1999) and Voss et al., J. App. Phys., Vol. 79, No. 2, pp.1170-1172 (January 1996).
Although eptaxial-side mounting is an established technology, several problems arise in applying it to QC lasers, since conventional semiconductor laser diodes and QC lasers differ in several important respects.
First, QC lasers usually are not planar because significant optical and current confinement is necessary to obtain a low threshold current and, in turn, a low dissipated power at the lasing threshold. Thus, QC lasers are commonly deep etched, ridge waveguide structures, with a ridge height .about.5 .mu.m. The side-walls of the ridge are typically covered with an electrical insulation layer (e.g., 300 nm of Si.sub.3 N.sub.4 or SiO.sub.2). On top of the laser ridge and the insulation layer typically 30 nm/300 nm of Ti/Au are deposited. This metallization has open gaps along the ridge to facilitate cleaving of the laser facets. The operating voltage of QC-lasers is typically .about.5-10 V. Therefore, even slight damage to the insulating layer--as can occur when the devices are cleaved--will cause a local breakdown of the insulation when a 5-10 V bias voltage is applied across it. Once the insulation layer is damaged, the unipolar ridge waveguide QC laser structure, which typically does not contain any current blocking layers, experiences relatively large parasitic leakage currents (after the devices are soldered to a heat sink).
Finally, the entire area of the ridge in a QC laser needs to be in good thermal contact with the heat-sink to avoid the occurrence of "hot-spots", where--as a consequence of the large amount of dissipated heat--devices tend to break down catastrophically. Therefore, the solder-bonding has to be extended to the very end of the laser ridge in a way that does not electrically short-circuit the active region (which is directly exposed at the facets) and does not otherwise contaminate the facet with solder (in a way that would cause unwanted scattering of the laser radiation). Most conventional diode lasers, which dissipate much less power, avoid this problem by not being soldered to the very end of the laser chip.
Similar considerations apply to other types of ISB lasers/emitters; e.g., superlattice (SL) QC lasers as described by Scamarcio et al., Science, Vol. 276, pp. 773-776 (May 1997), pre-biased SL lasers as described by Tredicucci et al., Appl. Phys. Lett., Vol. 73, No. 15, pp. 3101-2103 (October 1998) and non-cascaded, single stage ISB lasers described by C. Gmachl et al., Appl. Phys. Lett., Vol. 73, No. 26, pp. 3380-3382 (December 1998), all of which are incorporated herein by reference.
Thus, a need remains in the ISB laser art for a heat sink mounting approach that raises the temperature at which the lasers can operate cw, yet does so without increasing the risk of either short circuits at the facets or hot spots at the ends of the laser chips.